Morten Mostgaard Eldrup

Positronium formation at low temperatures: the role of trapped electrons

Abstract Measurements have been carried out of electron spin densities (by electron spin resonance technique) and positronium (Ps) formation probability as functions of Co-60 γ-irradiation dose in poly(methyl methacrylate) and linear poly(ethylene) at 77 K. We observe a linear relationship between the enhancement of the Ps formation and the density of trapped electrons in both polymers. This clear correlation strongly supports the previous suggestion by the authors that the increase in Ps formation with time (that has been observed at low temperatures for a number of polymers) can be explained as a reaction of free positrons with trapped electrons produced by the previously injected positrons.

Free-volume distribution and positronium formation in amorphous polymers: Temperature and positron-irradiation-time dependence

Positron annihilation lifetime spectroscopy was used to study the free-volume size distribution and the o-Ps (ortho-positronium) formation in two amorphous polymers. We performed positron lifetime measurements on poly(vinyl acetate) (PVAc) in the temperature range of 84–414 K and on poly(methyl methacrylate) (PMMA) in the temperature range of 84–454 K and as a function of time (<200 h) at four temperatures (T=84, 149, 224, and 249 K). The glass transition temperature Tg and secondary transition temperature were determined from the average o-Ps lifetime τ3 versus temperature. The width of the o-Ps lifetime distribution was evaluated using the maximum entropy method (MELT). Analysis by the MELT program on the spectra shows that (i) the standard deviation σ(τ3) of the o-Ps lifetime distribution in PVAc and PMMA increases slightly with temperature in the glassy state, and then increases significantly with temperature above the glass transition temperature Tg; (ii) the σ(τ3) in PMMA has no observable variation...

Bulk Metallic Glasses

Improved testing and a better understanding of the properties of bulk metallic glasses will lead to new avenues for commercial use.

Dose dependence of defect accumulation in neutron irradiated copper and iron

Abstract In order to investigate the difference in defect accumulation between fcc Cu and bcc Fe, tensile specimens were neutron irradiated at ≃70 °C in the HFIR reactor at Oak Ridge National Laboratory to fluences in the range of 4.5×1020–4.7×1024 n/m2 (E>1 MeV) corresponding to displacement dose levels in the range of about 0.0001–0.8 dpa. Irradiated specimens were characterized using positron annihilation spectroscopy, transmission electron microscopy and electrical conductivity measurements. A limited number of iron specimens were also tensile tested. At 0.0001 dpa, a low density of very small vacancy clusters (1–3 vacancies) were detected in iron, while bigger three-dimensional cavities were observed at higher doses. Both their density and average size increased with increasing dose level. In contrast, no such cavities were observed in copper. Irradiation led to an increase in yield stress and a decrease in the uniform elongation for iron.

Positron Lifetimes in Pure and Doped Ice and in Water

Positron lifetime spectra were measured in mono‐ and polycrystalline light ice, polycrystalline heavy ice, doped light ice, as well as in light and heavy water. All spectra were resolved into three components. At temperatures between −196° and −100°C the lifetimes and relative intensities are virtually constant, being 0.12 nsec and 20%, 0.44 nsec and 28%, 0.68 nsec and 52%, and attributed to annihilation of para‐Ps, free positrons, and ortho‐Ps, respectively. Above −100°C the longest lifetime increases with temperature to 1.1 nsec at 0°C. Its relative intensity is unchanged, while the parameters for the other components show a complex behavior. The spectra for mono‐ and polycrystalline light ice and for polycrystalline heavy ice are identical. For water long lifetime components attributed to ortho‐Ps are 1.86 nsec, 27% for H2O and 2.01 nsec, 22% for D2O. Theoretical explanations are suggested. Fast frozen solutions of HF with concentrations larger than approximately 10−5 mole fraction have a lifetime comp...

Effects of heat treatments and neutron irradiation on microstructures and physical and mechanical properties of copper alloys

Abstract Tensile specimens of CuAl 2 O 3 , CuCrZr and CuNiBe alloys were given different heat treatments and then irradiated with fission neutrons at 250°C to a dose level of ≅ 0.3 dpa. Both unirradiated and irradiated specimens were tensile tested at 250°C. The microstructure and electrical resistivity were determined in the unirradiated as well as irradiated conditions. The post-deformation microstructure and fracture surfaces were also investigated. The main effect of the bonding thermal cycle heat treatment was a slight decreased in the strength of CuCrZr and CuNiBe alloys. The strength of CuAl-25, on the other hand, remained almost unaltered. The post-irradiation tests at 250°C showed a severe loss of ductility in the case of CuNiBe alloy. The irradiated CuAl-25 and CuCrZr specimens, on the other hand, exhibited a reasonable amount of uniform elongation. The results are briefly discussed in terms of thermal and irradiation stability of precipitates and particles and irradiation-induced segregation, precipitation and recovery of dislocation microstructure.

On grain-size-dependent void swelling in pure copper irradiated with fission neutrons

Abstract The effect of grain size on void swelling has its origin in the intrinsic property of grain boundaries as neutral and unsaturable sinks for both vacancies and self-interstitial atoms. The phenomenon had already been investigated in the 1970s and it was demonstrated that the grain-size-dependent void swelling measured under irradiation producing only Frenkel pairs could be satisfactorily explained in terms of the standard rate theory (SRT) and dislocation bias. Experimental results reported in the 1980s demonstrated, on the other hand, that the effect of grain boundaries on void swelling under cascade damage conditions was radically different and could not be explained in terms of the SRT. In an effort to understand the source of this significant difference, the effect of grain size on void swelling under cascade damage conditions has been investigated both experimentally and theoretically in pure copper irradiated with fission neutrons at 623 K to a dose level of about 0.3 displacement per atom. ...

On recoil energy dependent void swelling in pure copper. Part I. Experimental results

Abstract In recent years, the problem of void swelling has been treated within the framework of the production bias model (PBM). The model considers the intracascade clustering of vacancies and self-interstitial atoms (SIAs), their thermal stability and the resulting asymmetry in the production of free and mobile vacancies and SIAs. The model also considers the influence of onedimensional diffusional transport of glissile clusters of SIAs on damage accumulation in the form of voids and defect clusters. One of the major predictions of the PBM is that, at a given irradiation temperature and damage rate, the void swelling should depend sensitively on the recoil energy, since it affects strongly the intracascade clustering of SIAs and vacancies, particularly at lower recoil energies. In order to test the validity of this prediction directly by experiment, pure and annealed copper specimens were irradiated with 2.5 MeV electrons, 3 MeV protons and fission neutrons at about 520 K. All three sets of irradiation experiments were carried out with a similar damage rate (of the order of 10−8 NRT dpas−1). Post-irradiation defect microstructures were investigated using electrical resistivity, transmission electron microscopy and positron annihilation spectroscopy. The accumulation of defects in the form of planar clusters and voids is found to increase substantially with increasing recoil energy. This is in good accord with the predictions of the PBM.

A Positron Annihilation Investigation of Defects in Neutron Irradiated Copper

Abstract The response of positron annihilation parameters to the defect structures produced by neutron irradiation in copper, and their annealing behaviour, have been studied on specimens irradiated at two different temperatures, 50°C and 250°C. Both lifetime and angular correlation measurements were made while some aspects of the irradiation damage substructure were also covered by transmission electron microscopy. Marked changes in the positron parameters were found after the two irradiations. After the 50°C irradiation the dominant TEM defect was a high concentration of small loops, after the 250°C irradiation it was a population of voids. The voids gave rise to a long-lived component (420 ± 75 psec) that disappeared during isochronal annealing to between 450T and 550T in agreement with TEM observations. Another trapped positron component (180 ± 7 psec) appearing after both irradiations annealed out between 275 ± 25°C and 475 ± 25°C. Although we could not entirely exclude that the defects giving rise t...

Annealing behaviour of copper and nickel containing high concentrations of krypton studied by positron annihilation and other techniques

Bulk copper and nickel samples containing 3 and 5 at. % krypton respectively have been studied by conventional positron annihilation techniques. Most of the emphasis has been placed on the changes in positron lifetime and angular correlation parameters during isochronal annealing from ambient up to near the metal melting points. The study was complemented by transmission and scanning electron microscopy, together with macroscopic measurements of weight and dimensions. These techniques, combined with previous studies, provide a fairly detailed picture of both the as-prepared materials, where the krypton is present as a high density of solid phase precipitates (solid bubbles), and the subsequent marked response of the substructure to annealing. A number of features are of particular interest. The onset temperature for bubble coalescence events is correlated with the melting temperature of the solid Kr inside the bubbles. At higher temperatures extensive swelling is observed prior to and simultaneously with the release of the majority of the krypton. The structure after the Kr release contains micrometre-sized pores and approximately 10 nm bubbles. This structure partly recovers at higher temperatures, but sufficient krypton is retained in the pores to maintain a large swelling up to close to the metal melting points. Positrons are found to become trapped at the Kr-metal interface in Kr bubbles, and there is clear evidence for a quantitative relation between lifetime and Kr density. Both these features are in agreement with the recent theory of Jensen and Nieminen (1987). Finally, positron trapping into dislocations is observed to occur at a rate much lower than that predicted from published specific trapping rates.